Solar cells are ideal electric energy generators, except in respect to initial installation cost, because during operation they consume no energy from terrestrial sources, such as fossil fuel or nuclear fuels, and therefore do not produce any pollution or create any undue hazard to public health and safety. Unfortunately, the present cost of producing solar cells and their low efficiency rating, makes solar electric energy too costly for use in any but the most exceptional cases, such as at remote microwave repeater stations or off-shore oil platforms. The current objective is to reduce the cost of solar electric energy to less than 70 cents per peak watt.
To reduce the solar electric energy cost, both conversion efficiency and cell fabrication costs may be improved, but neither should be improved at the expense of the other since the net gain may well be nil. Because research and development in this field is expensive, it is desirable to conduct research on a type of solar cell that will provide promise of improved cost and improved conversion efficiency as well. Of the known types of solar cells, GaAs single-crystal cells have the greatest potential for efficiency. The oldest type are Si single-crystal cells, but the production of large-area Si crystals continues to be a major problem with possible solutions that are too costly. Polycrystalline CdS cells are more promising in terms of cost, but reportedly have an efficiency of only 9%, and have stability problems. Research in amorphous silicon cells may eventually reduce production cost, but reported efficiency is only 5.5%, lower than for polycrystalline CdS. Polycrystalline GaAs cells may prove to be more efficient, but efficiencies that have been reported are about 8%. There is thus a need to develop a solar cell structure, and method of producing the structure, for high efficiency (about 18% or greater) and low cost.
The use of GaAs as the semiconductor material is widely recognized for obtaining high efficiencies on thin-film solar cells. This is partially due to its high light absorption and better match to the solar spectrum. Also, high efficiencies are to be expected because the barriers used in GaAs, such as n/p homojunctions, are reasonably stable at normal operating temperatures and do not have the problems of many heterojunctions. Such heterojunction problems, except for the InP/CdS configuration, arise from lattice parameter and electron affinity mismatch between the two components. These in turn can cause high interface recombination state densities and unwanted barriers in the conduction bands, respectively, leaading to reduced open-circuit voltages (V.sub.oc) and fill factors.